Poster of documentary "Masters in the Forbidden City".[Photo/IC]

After years of effort, the conservation department managed to reconstruct the recipe through chemical analysis.

However, even with all the right ingredients on hand, Qu and his team still had to wait, as the old, frail plaque and its frame had been partially damaged by expansion and contraction caused by heat.

"If it is restored now [in winter], thermal expansion may cause even greater damage next summer," Qu said.

Earlier this year, "Masters in the Forbidden City," a documentary series that profiled the work of the museum conservationists, became a surprise online hit. It has racked up over six million views and scored an impressive rating of 9.5/10 on a major video-streaming website in China.

A film version was released on Dec. 16, on the heels of the TV series' success.

The conviction, inner peace and persistence of the technicians have made them heroes among many of China's post-80s and post-90s generations.